Sodium/sulfur (Na/S) electrochemical cells or batteries are well known in the art and generally have at least one anodic reaction zone containing anodic reactant (sodium) and at least one cathodic reaction zone containing cathodic reactant (polysulfides, sulfur). The reaction zones are separated from each other by a sodium ion conducting solid electrolyte and the battery also has a current collector (i.e., electrode) in contact with the cathodic reactant. Such batteries operate at relatively high temperatures which accelerate the corrosion of battery materials which contact the sulfur and polysulfides. Thus corrosion is deleterious to the battery both from the standpoint of the resulting physical change of, e.g., the current collector (especially when it also serves as the cell container) and the reduction of the cell's capacity due to the reaction of the sulfur in the formation of corrosion products (i.e., cathodic reactant is used up). The corrosion products themselves form surface layers on the electrode that increase contact resistance to the electrode. Also, soluble corrosion products can be redeposited, clogging the electrode, obstructing transport of active materials, and causing uneven current distribution. If deposited on the electrode surface, they change its structure and wetability and therewith the kinetics of reactions occurring at this surface to the point where it may become partially or completely blocked. Corrosion products can also be deposited on the surface of the solid electrolyte partially blocking, damaging or destroying it by causing locally excessive current densities or by direct interaction or penetration.
In an attempt to overcome these corrosion problems associated with the sodium-sulfur battery, a variety of materials have been proposed for use in contact with sulfur and polysulfides in a Na/S battery. For example, in U.S. Pat. No. 3,413,150 proposes the use of aluminum with a coating of chromium or titanium or chromel (a nickel-chromium alloy) as a corrosion resistant material.
In U.S. Pat. No. 3,959,013, a proposal is made to use a corrosion resistant and electronically conducting layer of molybdenum or graphite over metal, e.g., stainless steel, to reduce the corrosion of such metals in a sodium-sulfur cell.
U.S. Pat. No. 4,048,390 suggests that a protective coating of aluminum be placed on the surface of the battery container which is to confine the polysulfide and sulfur reactants. This patent proposes the use of aluminum because it forms a continuous coating of aluminum sulfide over its exposed surface.
U.S. Pat. No. 4,110,516 suggests forming the confining container of aluminum and then placing over the aluminum either a single layer of chrome or a layer of zinc with a layer of chrome thereover, the chrome surface being the surface which contacts the corrosive reactants which are to be confined therewithin.
The following two patents attempt to overcome problems associated with the use of chromium or chromium alloys in a Na/S battery. U.S. Pat. No. 4,117,209 teaches a cathodic current collector comprising an aluminum substrate, a nickel-chromium alloy interlayer and an outer layer of an electronically conductive oxide which is intrinsically inert to the cathodic reactant, e.g., titanium oxide. British Patent No. 2,013,022 suggests a cathodic current collector for a Na/S battery comprising an aluminum substrate, a nickel-chromium alloy interlayer and an outer layer of a nickel oxide coating. In these two patents, the oxide outer layer is provided to offer protection to the interlayer against the cathodic reactant while providing electronic contact with the interlayer.
U.S. Pat. No. 4,131,226 discloses a cathodic electrode/container for a sodium sulfur battery in which a liner material of metal such as stainless steel, molybdenum or a nickel/chromium alloy is used as an anticorrosive surface for a mild steel container.
In U.S. Pat. No. 4,160,069, the current collector comprises a corrosion resistant ceramic member and an intimately attached metal cladding. The ceramics employed comprise doped futile TiO.sub.2, doped calcium titanate and lanthanum strontium chromite.
U.S. Pat. No. 4,166,156 teaches employing a current collector and/or container or liner of the cell or battery comprising a shaped graphite body bearing a coating of pyrolytic graphite on at least those surfaces thereof which are exposed to the cathodic reactant during operation of the cell. Such bodies may be made by compressing graphite into a shape and then pyrolyrically depositing graphite on or into the pores of the shaped graphite body. In contrast to the graphite/graphite electrode material of that invention, the electrode material of this invention is a carbon/carbon material woven of high tensile modulus carbon filaments in toe configuration impregnated with carbon particulate. Because the filaments of the electrode material are substantially continuous as compared to the graphite body which has pores, the carbon/carbon material has better electrical conductivity than the graphite/graphite bodies.
U.S. Pat. No. 4,216,275 attempts to overcome the corrosive nature of the polysulfide melt of a Na/S battery by providing a metal cell wall which is coated first with a prime coat of nickel and aluminum, and then applied on this prime coat is a coating of an alloy of chromium and at least one metal of the group of iron, cobalt and nickel.
U.S. Pat. No. 4,226,922 suggests that longevity of the cathodic current collector can be obtained if the metallic current collector has a boronized surface and an additional boron source in physical proximity to the boronized current collector surface.
Still another approach to forming a non-corrosive, electrically conductive component for a sodium sulfur cell is taught in U.S. Pat. No. 4,232,098. The component comprises a fiber-carbon substrate and a non-porous chromium-iron-carbon duplex alloy surface layer chemically diffusion bonded to the substrate. However, the component would be expected to have significant (series) resistance and degrade with high temperature operation.
In U.S. Pat. No. 4,248,943 a coating of chromium/chromium oxide is placed on the surfaces of the electrically conducting components of a Na/S battery to combat corrosion by molten sodium polysulfide and sulfur reactant.
In U.S. Pat. No. 4,279,943, stainless steel, fibrous or felt form, are proposed for use as a corrosion titanium nitride, graphite or carbon, especially in resistant cathodic electrode material in a Na/S battery.
Doped chromium oxide is taught in U.S. Pat. Nos. 4,456,631 and 4,456,664 to be useful as a coatings on metal or metal alloy so as to form corrosion resistant current collectors suitable for use in Na/S batteries. The patents teach lithia doped and magnesia doped chromium oxides, respectively.